The prior art discloses gas turbines (Field experience with the sequential combustion system of the GT24/GT26 gas turbine family, ABB Review 5/1998, page 12 et seq), in which the cooling air required for the turbine is passed in an MBH30 system through external cooling air ducts from the compressor into a separate cooling air cooler, re-cooled there and then made available to the turbine. The MBH30 system refers here to a secondary air system of the GT24/GT26 gas turbines, in which air branched off from the compressor after stage 16, which is at a pressure of approximately 20 bar, is used for cooling turbine components (mainly blades). The air exchanged in the cooling air cooler is passed on within a combined heat and power generating plant equipped with the gas turbine to the combined cycle and to the downstream steam circuit.
Such a known gas turbine is represented in a schematized form in FIG. 1 (see also U.S. Pat. No. 5,577,378, which is incorporated by reference herein). The gas turbine 10 of FIG. 1 comprises a compressor 11 for compressing the combustion air and discharging the compressed combustion air into a plenum 12, a first combustion chamber 13, which follows the compressor 11 and in which a fuel is burned while compressed combustion air is supplied, a first turbine 14, in which the hot combustion gases from the first combustion chamber 13 are expanded to perform work, a second combustion chamber 15 and a second turbine 16, arranged downstream of the second combustion chamber 15. Part of the compressed combustion air is branched off from the plenum 12 by means of a cooling air duct 17, cooled in a cooling air cooler 18 and used for cooling the gas turbine.
For safety and operational reasons (for special operating conditions), a pressure reserve is provided in the design of the cooling system. By means of a throttling device 19 in the form of orifice plates in the cooling air duct 17, the pressure is throttled back to the target pressure (cooling air pressure pcool) required in the turbine. Excessive pressure in the turbine cooling system would result in an unnecessarily great amount of cooling air being used. The throttling is checked during commission and if need be set to the machine (by appropriate choice of the orifice plates). However, throttling entails loss of exergy, which could otherwise be used in expansion.